Electromechanical wave transmission systems



,Nov. 13, 1962 w. P. MASON ELECTROMECHANICAL WAVE TRANSMISSION SYSTEMS Filed Aug. 14, 1959 2 Sheets-Sheet 1 FIG. 2

PRIOR Akr -TRAN$DUCER' FILTER INVENTOR BY W. P. MA SON #KQW' ATTORNEY Nov; 13, 1962 W. P. MASON ELECTROMECHANICAL WAVE TRANSMISSION SYSTEMS Filed Aug; 14, 1959 FIG. 5 3 2c;

III 2 3 2 Sheets-Sheet 2 SVMMETR/CA L SECTION FILTER FIG. 8

m/s/avrw WRMASON ATTORNEY States Patent Gfifice 3,064,213 Patented Nov. 13, 1962 3,064,213 ELECTROMECHANICAL WAVE TRANSMKSSIGN SYSTEMS Warren E. Mason, West Orange, N.J., assignor to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Filed Aug. 14, 195?, Set. Jo. 833,816 2 Claims. (Qi. 333-71) This invention relates to apparatus and methods for improving the characteristics of oscillatory wave energy transmission systems. More particularly, the invention relates to systems in which one or more transducers effecting conversion of the oscillatory wave energy from electrical to mechanical form and/or vice versa are employed in combination with a mechanical wave filter mechanically connected to the transducer, or transducers.

In prior art systems of the above-indicated type a frequently encountered and often most inconvenient limitation is the relatively restricted predetermined band of frequencies of the oscillatory wave energy which can be freely transmitted. As is well known to those skilled in the art, this limitation is in the usual case primarily a result of the fact that substantially all forms of electromechanical transducers are, as individual units, inherently capable of freely transmitting only a relatively limited frequency band of oscillatory wave energy. While a substantial widening of the frequency band which is freely transmitted by the transducer can be realized by placing an electrical impedance, as for example, an inductance, in series or in shunt with the electrical terminals of the transducer, in accordance with the principles Well known and widely used by those skilled in the art, the improvement effected by this measure alone is not sufiicient in instances where appreciably wider predetermined frequency pass-bands are required.

A further inconvenience frequently encountered in electromechanical transmission systems of the prior art is the bulk and Weight of the mechanical wave filters.

It is, accordingly, a principal object of the invention to substantially widen the predetermined band of frequencies of the oscillatory wave energy which can be freely transmitted while effectively suppressing the transmission of all other frequencies through a system which includes one or more combinations comprising an electromechanical transducer and a mechanical wave filter mechanically interconnected with the transducer for the transmission of the predetermined frequency band of oscillatory wave energy between the transducer and filter.

A further object is to reduce the bulk and Weight of mechanical wave filters.

The above principal object is realized in accordance with the principles of the present invention by mechanically interconnecting the electromechanical transducer and mechanical wave filter in such manner that a substantial portion of the so-called series impedance of the transducer itself is in effect utilized as a predetermined specific integral portion of the associated mechanical wave filter. The combination of filter and transducer can then freely transmit a substantially wider band of frequencies of the oscillatory wave energy and still effectively suppress all other frequencies. A further increase in the width of the frequency band which can be freely transmitted by the combination of transducer and filter is efiected in accordance with the principles of the invention by employing an impedance transforming mechanical filter section immediately adjacent the transducer.

The above-mentioned further object of the invention is effected as an inherent benefit resulting from reduction of the characteristic impedances of mechanical wave filters of the invention as described in detail hereinunder.

The principles of the invention will be more readily 2 understood from a perusal of the following detailed description of specific illustrative embodiments given hereinunder and form the accompanying drawings in which:

FIG. 1 illustrates a representative electromechanical structure designed in accordance with the teachings of the prior art and employed in the present application for the purpose of comparison with structures of the present invention;

FIG. 2 is an equivalent electrical schematic diagram of the structure of FIG. 1;

FIG. 3 illustrates an electromechanical structure embodying certain principles of the invention;

FIG. 4 is an equivalent electrical schematic diagram of the structure of FIG. 3;

FIG. 5 is an electrical schematic diagram employed in explaining further principles of the invention;

FIG. 6 illustrates an electromechanical structure embodying further principles of the invention;

FIG. 7 is an equivalent electrical schematic diagram of the structure of FIG. 6; and

FIG. 8 indicates the electrical circuit connections to the transducers as employed in all of the structures of FIGS. 1, 3 and 6.

In FIG. 1 a conventionally arranged combination of an electromechanical transducer mechanically interconnected with a mechanical wave filter is indicated by way of example and for purposes of comparison with combinations of the invention. The transmission bandwith (that is, the width of the pass-band, or range of frequencies freely transmitted 'by the combination with the substantial exclusion of all other frequencies) of this combination can be increased by from 35 to 40 percent by application of the principles of the present invention, as will be described in detail hereinunder.

While the combinations selected in this application for illustrative purposes are of a specific type and are designed to transmit torsional oscillatory mechanical wave energy, it will be obvious to those skilled in the art that the principles of the invention are applicable with equal facility and similar beneficial results to numerous differing forms of combinations designed to transmit longitudinal, shear or flexural oscillatory mechanical wave energy, as well as to other combinations of several differing types in which torsional oscillatory mechanical wave energy is employed.

In more detail in FIG. 1, the elongated transverse me. bers 24, 28, joined by short shaft members 26, comprise a portion of a band pass torsional mechanical wave filter of the generic type disclosed and claimed in the copending joint application of R. N. Thurston and applicant, Serial No. 564,682, filed February 10, 1956. This joint application matured as Patent 2,906,971 granted September 29, 1959. Filters of the present application difi'er specifically from the filters of the joint application in that the trans verse members of the filters of the above-mentioned joint application are discs or spiders instead of simple fiat rectangular transverse members.

An advantage of using simple flat rectan ular members as illustrated in FIGS. 1, 3 and 6 is that the whole filter can be stamped in one piece from a single sheet of resilient material, for example steel, which stamping can also obviously include the central conductive plate 15 of transducer 14 (see FIG. 8).

Transducer 14 can, for example, be of the type disclosed and claimed in the copending joint application of R. N. Thurston and applicant, Serial No. 803,007, filed March 30, 1959, and illustrated by FIG. 3 of the drawings accompanying the joint application. This joint application matured as Patent 3,004,176 granted October 10, 1961. The electrical connections to transducer 14 are shown in detail in FIG. 8 and will be described hereinund 1. This same transducer having the same electrical 'lations of the transducer.

connecting circuit as shown in FIG. 8 is employed in all of the illustrative combinations of the present application and is of a double Bimorph type. The structure of the transducer 14 is, in general, antisymmetric about its transverse axis as described in the joint application. On each side of the transverse axis paired piezoelectric or electrostrictive members are mounted. One member of each pair expands and the other contracts under the application of a particular polarity or half cycle of an alternating current electric signal. The members of each pair reverse their respective actions with each successive half cycle of the signal to flex the respective halves of the member in phase opposition so that an oscillatory torsional vibration is imparted to the shaft 22, 26.

Transducer 14 in FIG. 1 is mechanically connected by a very short section 22 of the shaft to the first transverse member 24 of the filter. (An electrical circuit as il. .5- trated in FIG. 8 is connected to transducer 14 but is not shown in FIGS. 1, 3 and 6 since it would tend to obscure mechanical features.) Shaft section 22 is made just sufficiently long to permit member 24 to vibrate in fiexure about its transverse axisin response to the torsional oscil- This is necessary since the transducer'14 shunted by inductance 29 (of FIG; 8) should resonate at the mid-frequency of the transmitted band while/element 24 should resonate at the lower cutoii frequencyof the pass-band. For reasons which will be explained in detail presently, the width of the first transverse member 24 is made one-half the width of subsequent transversemembers 28 and provides a mid-series arm and mid-series termination for the upper end of the mechanical filter. Above the transducer 14 a resilient supporting member 12 comprising for example a short piece of steel wire approximately ten mils (.010 inch) in diameter is centrally attached to transducer 14 and in turn is attached at its upper end to rigid support 10, as shown. The member 12 in addition to supporting the transducer adds a small increment to the impedance of r the. transducer.

I r shunted acros the electrical terminals 16 and 13 which are connected electrically to the transducer conductive electrodes 17 and central conductive plate 15, respectively, of the transducer. A pair of the electrodes 17 are provided on the outer surface of each of the piezoelectric, orelectrostrictive, members 13. but a small central portion of the surface as indicated in FIG; 8. The transducer assembly can be fabricated and preconditioned in any of the several ways described in my above-mentioned joint application, Serial No. 803,6G7. inductance 20 serves to resonate with the interelectrode capacity of the transducer at the mid-frequency of the transmitted (or pass) band of frequencies of the transducer. The efiect of such an inductance, as is well known in the art, is to produce an appreciable increase in. the bandwidth of the transducer.

nr the usual case the filter, in addition to the small end. Such an arrangement, ofcourse, employs the. second transducer to reconvert the mechanical wave energy transmitted to it through the filter into corresponding electrical wave energy. The duplication in and complica tion of the drawings which would be required to show the Electrodes 17 cover all structure in full are however not'considered necessary as it is believed that the principles of the invention are adequately illustrated by the portion shown. The second or lower transducer is preferably connected by a second member like member 12 to a rigid support like support 10 to prevent lateral movement of the whole assembly about the point of attachment to upper support 10.

In some instances the far end transducer may be, for

example, a sound recording stylus carriage or mechanical pickup, or the like, but those skilled in the art will have no difficulty with the application of the principles of the invention to such arrangements since they obviously can be essentially identical with those to be described hereinunder. a

It will become apparent as the description proceeds tha the above remarks in the two immediately preceding paragraphs also apply to the other structures described in this specification a illustrated in FIGS. 3 and 6 of the drawings; H

In FIG. :2 an equivalent electrical schematic diagram of the structure illustrated in FIG. 1 is given. Inductance 20 represents the shunting inductance 20 of FIG. 8. Capacitance C is the shunt capacity of the transducer 14. The combination of inductance L in series with capacitance C represents the effective series impedance of transducer 14 contributed by its inertia and compliance 7 V (or stiflness), respectively, including that of supporting member 12, so that, in accordance with conventional analysis of elecro-mechanical transducers by those skilled in the art the transducer'14 and its shunting inductance 20 can and usually are considered to be equivalent electrically ,to an electrical wave filter half-section terminated on its right by a mid-series termination, i.e. by a series arm L 0 having precisely half the impedance of a full series arm of a so-called laddeftype multisection filter of the same type.

'The terminology employed throughout this application in connection with electrical wave filter structures .or equivalent electrical schematic diagrammatic representations of'mechanical wave filter structures will conform generally to that employed for some decades in the electrical and electro-mechanical Wave filter arts. The terminology applicable to electrical wave filters and diagrams of equivalent electrical structures is exemplified, by way of example, in O. I. Zobels article entitled Theory and De-' sign of Uniform and Composite Electric Wave Filters published in the Bell System Technical Journal for January 1923, and in the textbook by T. E. Shea entitled I entitled Transmission Networks and Wave Filters published'by D. Van Nostrand Co., Inc., New York, New York, 1929, as well as in many other publications which have long been well known and extensively used by those skilled in this art; Section '37 at pages 175 through 177 of Sheas book is of particular interest in that it explains the relationships between T, 11' and L type sections of a ladder type filter. I I

The application of electrical wave filter theory to mechanical wave filter structures and the analogous nature of numerous and varied mechanical structures and structural elements with their respective electrical counterparts have also long been well known and extensively employed'by those skilled in the art. Applicants book Electromechanical Transducers Filters published by D. Van Nostrand Co., Inc., New

York, New York, 1948 (Second Edition), is, by way of specific example, a comprehensive text for the electromechanical and mechanical wave filter and related transmission arts. 7 V

The portion of the combination comprising the mechanical wave filter per se, as distinguished from the transducer 14, and consisting of transverse members 24, 28, and connecting shaft members 2601 FlG. 1, is represented in the equivalent electrical schematic diagram. of FIG. 2 by the ladder type electrical wave filter structure having full series arms each consisting of an inductance and Wave 7 L and a capacitance C connected in series (representing a full transverse arm 28 of FIG. 1), alternating with full shunt arms each consisting of the capacitor C (representing a shaft member 26 of FIG. 1). By analogy with the equivalent electrical schematic diagram, mechanical wave filters of the types illustrated in the accompanying drawings can aptly be designated as of the ladder type, the shafts representing shunt arms and the transverse members representing series arms, respectively. A midseries termination, represented in FIG. 2 by inductance /2L in series with capacity 2C is provided at the upper end of the mechanical filter per se. The impedance Z of FIG. 2 represents the very large shunting impedance of the very short section of shaft 22 and is of such large magnitude that it can be ignored for all practical purposes.

The filter elements are proportioned, in accordance with long established principles of electrical and analogous mechanical wave filter design, to have the same characteristic impedance and same transmission band as the equivalent filter section representing the transducer 14 of FIG. 1 (consisting of inductances 2t} and L and capacitors C and C of FIG. 2 as described in detail above). Such being the case, the impedance of the mechanical filter at its point of connection with the transducer will match the transducer impedance throughout the entire transmission or pass-band.

The Zobel article and Section 37 of Shcas book, both mentioned above, explain how any desired number of predetermined filter sections and sections of related types of filters may be combined to form a unitary ladder type filter structure. Analogous mechanical wave filter structures of numerous and varied types are readily devised and interconnected as taught, for example, in my abovementioned book.

The structural combination, as illustrated in FIG. 1, and electrically driven as illustrated in FIG. 8, is limited as to the width of the band of frequencies it will freely transmit by the width of the band which the electromechanical transducer 14 in combination with its shunt coil 20 will transmit.

In accordance with the principles of the present invention, however, it is possible by simple modifications of the mechanical wave filter structure associated with the transducer to produce combinations of an electromechanical transducer and a merchanical wave filter which will transmit or pass an appreciably wider band of frequencies than can be transmitted, or passed, by the transducer when joined to a mechanical filter in accordance with the conventional method illustrated by FIG. 1 and described in detail above.

It can be shown that the width of the band of fre quencies which the transducer, as a unit, will freely transmit is dependent upon the ratio of capacity C to capacity C where, as described in connection with FIG. 2 hereinabove, C is the capacity measured between the electrical terminals of the transducer and C is the electrical capacity equivalent to the mechanical compliance of the transducer.

Stated in another Way, if the effective value of capacitance C of FIG. 2 could in some way be increased, the ratio of C divided by C would be increased and the transducer would transmit a wider band of frequencies.

A fundamental concept of the present 1nvent1on is in effect that the transducers series arm, consisting of inductance L and capacitance C in series (as illustrated in the equivalent electrical schematic diagram of the transducer, FIG. 2) should not be considered just a midseries (or half of a series) arm as is done in connection with the conventional combination of FIG. 1, illustrated schematically in FIG. 2. Rather, L and C in series should be considered as a full series arm consisting of one half or mid-series arm which is associated with the shunt arm of the transducer comprising C and inductance 20 in parallel and a second half or mid-series arm which is in efiect appropriated to serve as the initial half or mid-series arm for the first section of the mechanical wave filter. In other words, the portion of the series impedance of the transducer to the left of broken line 54 of FIG. 4 provides a half or mid-series arm to be associated with the shunt arm (consisting of inductance 2t; and capacitor C of the transducer) in forming the equivalent of an electrical filter half section to represent the effective electrical equivalent of the transducer while the portion to the right of line 5:;- of FIG. 4 serves as the initial half or mid-series arm of the mechanical wave filter. Of course, these two portions of the series impedance of the transducer cannot be physically separated from each other but it is obviously only necessary that the full series impedance of the transducer be equivalent to the combined impedance of the two above-identified half or mid-series arms connected electrically in series and to interconnect the transducer and filter by an element of the v lter which corresponds in the equivalent electrical schematic diagram to a full shunt arm for the filter.

Referring to the equivalent electrical schematic diagram of FIG. 4, we can then represent the whole series arm of the transducer as consisting of two capacitors C and 2C and two inductances and L all four elements being connected in series, the four elements being the electrical equivalent of the original series combination of capacitor C and inductance L of FIG. 2. It can be shown that the portion of the transducer represented t the left of line 5% in FIG. 4 by the elements C C L /2L and inductance 2i is a halfsection of a filter which will freely transmit a band of frequencies approximately 35 percent wider than the band which will be freely transmitted by the whole transducer as represented by elements C L C and inductance 2%} of FIG. 2.

Since the filter and transducer are to transmit the same frequency band and match impedances throughout this frequency band, half of the original inductance L, can be assigned as being the inductance /2L where L is the inductance required for a full series arm of the new filter. The new filter as shown in FIG. 3 will differ markedly from the filter of FIG. 1 and the schematic diagram of FIG. 2 because it is to pass a 35 percent Wider band of frequencies, and in view of the effective reduction of the series impedance of both the transducer and the new filter it must have a lower characteristic impedance. The band will have the same mid-frequency a for the former filter.

The remainder of the original inductance L that is L /2.Z must resonate with capacity C at the midfrequency of the pass-band. This relation fixes the value of capacity C Capacity C can be shown t be where C is the original capacity as per FIG. 2. Capacitor 2C is, of course, the capacity required for the half or mid-series arm for the new filter (being in effect appropriated from the whole series impedance of the transducer to function with the new filter). Capacitor 2C must resonate with inductance /2L at the lower cutofi of the pass-band of the mechanical filter. The capacity of C must, obviously, be approximately twice that of the original capacity C since C in series with 2C must be equivalent to the single original capacity C and therefore the ratio of C to C is substantially greater than that of C to C and accordingly, as postulated hereinabove, a'widerpass-band will be realized.

Considering the combination consisting of a shunt arm comprising inductance 2i and capacitor C in parallel and a series arm comprising a capacity C and inductance L /2L in series, the pass-band and impedance of the portion of the transducer to the left of line 50 in FIG. 4 which is not assigned for appropriation for use as the initial series arm of the new filter can be readily determined and the design of the new filter can then be completed, i.e. the value of capacitor C can be determined, in accordance with principles well known to those skilled in the art. The new filter will provide the same pass-band and have the same characteristic impedance as the above-mentioned portion of the transducer which is not appropriated for use as part of the new filter.

Since the initial series arm of the new filter is provided by the above described appropriation from the series impedance of the transducer, the first actual arm of the new filter will be the mechanical equivalent of a full shunt arm, consisting of a capacitor C as represented in the electrical schematic diagram of FIG. 4.

The full series arms of the new filter will, in like manner, each consist of the mechanical equivalent of the combination of an inductance L and a capacitance connected inseries, as shown in FIG. 4. These full series arms will also, of course, be resonant at the lower cutk ofi of the pass-band of the filter.

The overall mechanical structure corresponding to the diagram of FIG. 4 is then as shown in FIG. 3. The transducer 14 is the same as that used in FIG. 1 and is electrically connected as shown in FIG. 8. It is mechanically connected to a section of shaft 32. Shaft 32 is of the length required to produce the mechanical equivalent of the above-mentioned full shunt capacitor C The actual filter structurefaccordingly, will comprise alternately a section 32 of shaft and a transverse member 34, as shown. Transverse members 34 of FIG. 3, 'of course, are each the mechanical equivalent of the full series arm L C of the new filter as represented in the equivalent electrical schematic diagram of FIG. 4. If stamped from material of the same kind and thickness as was used for the filter of FIG. 1, transverse members 34 will be slightly less than half the width of transverse members 28 because of the lower characteristic impedance but will be slightly longer than members 28' since they must be resonant, for the specific type of'filter shown, at the new lower cutoff frequency, that is at the lowest frequency of the newer and wider band freely transmitted by the combination of the transducer and the new filter.

V Manifestly the filter structure of FIG. 3 will be appreciably lighter in weight and less bulky than that of FIG. 1. By way of specific example, where a structure of the 7 type illustrated by FIG. 1 can transmit the frequency trated by FIG. 3 can transmitjthe frequency band of approximately 6,8 93 to 8,108 cycles per second (a 16.2 percent band centered about a mid-frequency of 7,500 cycles per second). Thusthe novel combination illustrated in FIG. 3 and represented by the equivalent electrical V schematic diagram of FIG. 4 provides substantially a 35 percent wider band of freely transmitted frequencies than does the combination of FIG. 1. 7

An. additional increase of approximately '5 percent in bandwidth and a further marked reduction in the bulk and weight of the filter over that provided by the combination of FIG. 3 can be realized by introducing an impedance transformation in'the filter section immediately adjacent to the transducer, as will be described in detail hereinunder. V

A filter section is said to be an impedance transfoming section when it has substantially differentimpedances at its two ends.

As is well known to those skilled in the art, whenever a filter section is so constituted that it can be represented schematically as having a T of three capacities, i.e. a capacity in each series arm and a capacity in the shunt arm, as shown for example in the symmetrical section to he righ i F G. 5.. n. impedance ansfcnnation can be in u y mo ifying he alues of t e three apa i i s an one of the seri rm nduetance hi ype section is found in numerousand' varied filters and occurs generally for the sections of all the filters represented by equivalent electric schematic diagrams in the present ap-: plication if sections terminated in mid-series arms at each end are separated out.) The procedureis illustrated, for example, in FIG. 181 at page 331 and described on' page 332 of T. E. Sheas above-mentioned book. If carried to its limit, this process can result in the complete elimination of one series capacity and substantial reductions in the magnitude of the two remaining capacities of the section. The results' of the procedure carried to the point where one series capacity is eliminated are illustrated by the impednace transforming section shown at the left in FIG. 5. V

The right-hand section of FIG. 5 is an unmodified or symmer-tcial section having like mid-series terminations on both ends, i.e., having two half series arms each consisting of a capacitor 2C and an inductor /2L and a single full shunt arm consisting of a capacitor C Since it is a symmetrical section, the impedances at both ends of the section are the same. The designations are double primed in FIG. 5 to indicate that the filter has been designed to pass a 5 percent wider hand than the corresponding filter of FIG. 4.

The impedance transforming section has, of course, been subiected to the above-described impedance'transforming procedure and the capacitor has thus been entirely eliminated from its right series arm. Its shunt capacitor has been reduced in capacity to and the left series arm now comprises an inductor having, an inductance of 2031!! owl) Where Q is the impedance transformation ratio. .By way of example, a transformation ratio of substantially 33.8 was foundto just eliminate the capacitor in one series arm in a specific case; 1

Notwithstanding the elimination of the capacitor from the right armof the impedance transforming section of FIG. 5, the impedance at the right end of the section is the same as that of the symmetrical section to its right and can therefore be connected directlyto the mid-series termination of the second section as indicated by the dash lines between the respective terminals, without incurring; reflection losses. 7 r v a The impedance at the terminals of the left endof the. first section, however, is increased by the transformation. ratio, i.e. by P r V V Of more direct interest for the purposes of the present invention is the fact that the capacity of the capacitor in the left series arm of the impedance transforming section has been reduced by the factor 9 from the transducers series impedance for use as part of the filter have the values then there will be left, as the equivalent electrical series impedance of the transducer, the capacitor and the inductor structure substantially as represented by the equivalent electrical schematic diagram of FIG. 7 where the filter of FIG. 4 is replaced by a similar filter of 5 percent greater bandwidth having a first shunt arm on the left of and an. adjacent series arm to its right comprisnig the series combination of inductor L and capacitor 2C Subsequent shunt arms are C and subsequent series arms are L and C in series, as shown. The portions of the impedance transforming section contributed by the filter per se are of course the extreme left shunt arm of the filter comprising capacitor and the first series arm of the filter per se, that is the series combination of inductor L and capacitor 2C The initial series arm appropriated from the series impedance of the transducer to act as an initial arm of the filter is, of course, a portion of the series impedance of the transducer as described in detail above.

A structural combination corresponding to the equivalent electrical schematic diagram of FIG. 7 is illustrated in FIG. 6. It includes transducer 14 supported by short resilient element 12 from rigid support 10, as for the prior combinations illustrated by FIGS. 1 and 3 of the drawings. In the mechanical filter per se, however, a first section 36 of shaft is connected to transducer 14 and is proportioned to provide the mechanical equivalent of the initial shunt arm of the impedance transforming section represented in the diagram of FIG. 7 by capacitor Coll The first transverse element 33 is the mechanical equivalent of the adjacent filter series arm represented by the series combination of inductor L and capacitor 2C Subsequent shaft sections 40 of the filter then represent the mechanical equivalents of full shunt arms each comprising a capacitor C of the diagram of FIG. 7. Similarly, subsequent transverse elements 42 of the filter represent the mechanical equivalents of full series arms each comprising the series combination of an inductor L and a capacitor C of the diagram of FIG. 7.

To obtain dimensions of the shaft portions and transverse members of the mechanical filter of FIG. 6 which are somewhat comparable with the corresponding dimensions of the corresponding portions of the filter of FIG. 3, it was found necessary (in view of the impedance transformation introduced by the first section of the filter of FIG. 6) to reduce the thickness of the mechanical filter of FIG. 6 to a third of the thickness of the filter of FIG. 3 and to reduce the width of the shaft portions to one-half that of the filter of FIG. 3. Otherwise shaft portions and transverse members of impracticable dimensions subject to vibration in numerous unwanted modes would h-ave resulted. (For example, shaft portions more than thirty times the length of those of FIG. 3 and extremely narrow transvers members would have been required.)

It is thus apparent, since FIGS. 1, 3 and 6 all represent actual designs all drawn to the same scale (all dimensions are tripled, i.e. the scale is three to one), that the introduction of the impedance transforming section at the junction between the transducer and the mechanical filter as illustrated in FIG. 6 not only provides a freely transmitted band approximately 5 percent wider than that of the combination of FIG. 3 but also results in a very substantial reduction in the physical bulk of the mechanical filter. This, of course, is primarily of interest at lower frequencies where the component parts of the mechanical filter tend to become inconveniently large.

To recapitulate,the combination of FIG. 3 provides a freely transmitted band of frequencies substantially 35 percent wider than the band of the combination of FIG. 1 and an appreciably less bulky filter than that of FIG. 1.

The combination of FIG. 6 provides a further increase of substantially 5 percent in bandwidth and a very great decrease in the bulk of the filter even when compared with the filter of FIG. 3.

In both of the combinations of FIGS. 3 and 6 the initial mid-series arm of the filter is appropriated from the series impedance of the transducer so that the first actual filter element is a full shunt arm. In both combinations the filter is designed to freely transmit the respective wider frequency band which the combination is capable of transmitting. Furthermore, in both the combinations of FIGS. 3 and 6 there is no impedance mismatch at the junction between the transducer and the mechanical filter.

Numeous and varied other structures embodying the principles of the invention and within the spirit and scope thereof will readily occur to those skilled in the art. The above specific examples are illustrative only and are in no sense to be considered as limiting the invention or as exhaustively embracing all possible structures of the invention.

What is claimed is:

1. In combination, an electromechanical transducer, an electrical reactance electrically connected to the electrical terminals of the transducer and resonating with the electrical reactance of the transducer at the midfrequency of the pass-band of the transducer, and a mechanical band pass wave filter, the filter being mechanically connected to the transducer for the trans mission of oscillatory mechanical Wave energy between the transducer and the filter, the transducer and its associated electrical reactance constituting a band pass wave filter having a first characteristic impedance and a transmission band of a specific width centered about a specific mid-band frequency, the mechanical filter consisting of av plurality of mechanical elements arranged as a ladder type mechanical wave filter comprising alternate shunt and series arms, the mechanical wave filter having a characteristic impedance of substantially one-half that of the said first characteristic impedance and a pass-band substantially thirty-five percent wider than that of the transducer and its associated electrical reactance, the pass-band of the filter being also centered about the said specific mid-band frequency, the connection to the transducer being efiected by an element of the mechanical filter which is a full shunt arm of the mechanical filter.

2. In combination, an electromechanical transducer adapted to convert oscillatory electrical wave energy into mechanical wave energy of a predetermined type and vice versa, an electrical reactance electrically connected to the electrical terminals of the transducer and resonating with the electrical reactance of the transducer at the mid-frequency of the pass-band of the transducer, the transducer and its associated electrical reactance constituting a band pass wave filter having :a first characteristic impedance and a transmission band of a specific Width centered about a specific mid-band frequency, and a mechanical band pass Wave filter adapted to transmit oscillatory mechanical wave energy of the predetermined type and consisting of a plurality of mechanical elements arranged in a ladder type structure, successive elements coupling adjacent elements for the transmission of the mechanical wave energy alternately in series and in shunt relation, the mechanical connection between the transducer and the filter consisting of a full shunt element of the filter, the main portion of the mechanical filter having a characteristic impedance which is a small fractional part of the characteristic impedance of the transducer and its associated electrical reactance, the shunt arm of the filter connecting to the transducer and 12, the series of the filter connecting to this; shunt arm comprising a portion of an impedance transforming filter section proportioned and adapted to transform the characteristic impedance ofnthe main portion of the filter to an impedance which is substantially one-half that of the characteristic impedance of the transducer and its associated electrical reactance, the filter having a passband substmtially forty percent Wider than the pass-band of the transducer and its associated electrical reactance, the Wider pass-band of the filter being also centered about the said specific mid-band frequency.

References Cited in the file of this patent UNITED'STATES lATENTS Davies Mar. 29, 1938 

